The standard planetesimal model of terrestrial-planet formation is based on astronomical and cosmochemical observations, and the results of laboratory experiments and numerical simulations. In this model, planets grow in a series of stages beginning with the micrometer-sized dust grains observed in protoplanetary disks. Dust grains readily stick together to form millimeter- to centimeter-sized aggregates, some of which are heated to form chondrules. Growth beyond meter size via pairwise sticking is problematic, especially in a turbulent disk. Turbulence also prevents the direct formation of planetesimals in a gravitationally unstable dust layer. Turbulent concentration can lead to the formation of gravitationally bound clumps that become 10-1000-km planetesimals. Dynamical interactions between planetsimals give the largest objects the most favorable orbits for further growth, leading to runaway and oligarchic growth and the formation of Moon- to Mars-sized planetary embryos. Large embryos acquire substantial atmospheres, speeding up planetesimal capture. Embryos also interact tidally with the gas disk, leading to orbit modification and migration. Oligarchic growth ceases when planetesimals become depleted. Embryos develop crossing orbits, and occasionally collide, leaving a handful of terrestrial planets on widely spaced orbits. The Moon probably formed via one such collision. Most stages of planet formation probably took place in the asteroid belt, but dynamical perturbations from the giant planets removed the great majority of embryos and planetesimals from this region.
- Pub Date:
- December 2010